The present invention relates to temporary protective coating compositions which are useful in passivating untreated metallic substrates. More specifically, the invention relates to aqueous temporary protective coatings having solution polymerization solvents that may be rapidly evaporated at relatively low drying temperatures and yet do not create a fire hazard due to low flash point of the composition.
Passivation of metallic substrates in mills is typically done with mill oils or chemical treatments in order to prevent or reduce corrosion, particularly white rust. Usually passivating agents are removed after the metallic substrates are formed into a part. Mill oils are difficult to remove and less than desired corrosion protection is provided thereby. Chemical treatments, particularly those involving film-forming materials, are usually incompatible with other materials and processes that are subsequently applied to the subject metallic substrates.
Art-related protective coating compositions comprising alkali-soluble carboxyl group-containing polymers and/or waxes are known in the art. However, most of these compositions are employed distinctly on painted or polished surfaces and are less effective on untreated metallic substrates. In contrast, the protective coating compositions which are of interest here should be suited to the application to untreated metallic substrates. Additionally, these protective coating compositions should be compatible with the subsequently applied materials and processes. Moreover, the passivated metallic substrates should be formable and weldable and cleanable with an aqueous alkaline solution, and should be corrosion resistant. These types of temporary protective coating compositions are provided by the inventions in the related application identified above.
The aqueous, polymeric, temporary protective coatings of the related applications employ organic solvents to carry out the preferred method of solution polymerization. The organic solvent component of the temporary coating composition must be volatile in order to be evaporated quickly from the coating so as to permit rapid drying after application onto the metal surface. Coating compositions of this type that include common solvents such as propylene glycol monomethyl ether and ethylene glycol monobutyl ether attain optimum coating properties when baked at temperatures in the range of 300.degree. F. to 450.degree. F. For the application of temporary coatings to moving steel strips, it would be desirable for the bake temperatures to be considerably lower, such as on the order of 200.degree. F. to 250.degree. F. (peak metal temperature).
It is also desirable that the solvents in temporary coatings of this type impart to the coating composition a relatively high flash point so as to reduce any fire hazard associated with the liquid compositions. Substances having flash points above 100.degree. F. are generally considered safe with regard to fire hazard, and it would be desirable for temporary coating compositions of the type involved here to exhibit that characteristic. However, attaining this objective is normally at odds with the objective of selecting solvents to provide rapid drying. It is also desirable that solvents meet the requirements of the solution polymerization process.
U.S. Pat. No. 5,006,578 (Masuda et al.) discloses polymerization of an acrylic resin in isopropanol for use in a temporary protective coating. The isopropanol is stripped from the resin prior to formulation of the coating composition. A lubricant such as wax is not included in the composition disclosed in the patent.